Rapid Thermal Processing (RTP) systems are increasingly being used for microelectronic device fabrication. As is well known to those having skill in the art, rapid thermal processing systems attain a desired processing temperature rapidly, without the need for a lengthy "ramp up" period. It has been found that rapid thermal processing systems allow microelectronic devices to be fabricated at high temperatures without causing dopant diffusion or other unwanted side effects. Since rapid thermal processing systems typically process semiconductor wafers, the term "wafer" will be used herein to designate any device processed in the rapid thermal processing system. It will be understood by those having skill in the art that other substrates or materials may be processed.
In contrast with a conventional furnace which typically uses resistive heating units, a rapid thermal processing system typically uses radiant heat sources, for example arc lamps or tungsten-halogen lamps. A small heating chamber is typically used, to provide a controlled environment for the wafer to be processed and to efficiently couple the heat energy from the radiant energy sources to the wafer.
Rapid thermal processing systems typically employ a pyrometer for sensing the temperature of the wafer. As is well-known to those having skill in the art, a pyrometer is a radiation sensitive thermometer which provides contactless temperature sensing by measuring emitted radiation from the wafer at a particular radiation wavelength and at an assumed wafer emissivity. Pyrometers have been used in temperature control systems for rapid thermal processors to maintain constant wafer temperature. In a temperature control system, a controller is employed in a feedback configuration to regulate the radiant heat sources based upon deviations of the measured pyrometer temperature from a desired wafer temperature. The design and construction of rapid thermal processing systems are well-known to those having skill in the art and are described, for example, in U.S. Pat. No. 4,115,163 to Gorina et al. and U.S. Pat. No. 4,755,654 to Crowley et al.
In the semiconductor manufacturing art, rapid thermal processing systems have been heretofore used for rapid thermal annealing and rapid thermal oxidation of semiconductor wafers. See for example U.S. Pat. No. 4,331,485 to Gat. In an annealing or oxidation process, it has been found that appreciable changes in the radiation properties of the wafer are not produced, so that the pyrometer measurement may be used to accurately control the radiant heat sources.
Attempts have recently been made to use rapid thermal processing systems for depositing layers on a semiconductor substrate, for example in a rapid thermal chemical vapor deposition (RTCVD) process. One thin film deposition which is widely used in microelectronic device fabrication is the deposition of a thin film of polycrystalline silicon on a silicon dioxide layer on the surface of a silicon wafer. Unfortunately, it has been found that a thin film deposition often dramatically changes the radiative heat transfer properties of the wafer, and thereby causes erroneous pyrometer readings, leading to unacceptable process variations in the RTCVD process. For example, when an opaque polycrystalline silicon film is deposited on the transparent silicon dioxide layer at the silicon wafer surface, the emissivity of the wafer changes dramatically as its surface changes.
The above-described changes in wafer emissivity as a function of a deposited material have been widely reported in the art. For example, in a publication entitled "The Effect of Thin Dielectric Films on the Accuracy of Pyrometric Temperature Measurement" published in the Materials Research Society Symposium Proceedings, Vol. 52, pages 209-216, by D. W. Pettibone et al., 1986, it is reported that in RTCVD, the thin films being deposited may impact the emissivity of the wafer. Pettibone et al. conclude that pyrometer recalibration is necessary to accurately measure wafer temperature in such a process. However, there is no suggestion as to how pyrometer recalibration may be accomplished.
Similarly, a publication entitled "Pyrometric Emissivity Measurements and Compensation in an RTP Chamber" published in the Spring, 1989 Proceedings of the Symposium on Rapid Thermal Annealing/CVD and Integrated Processing by Nulman et al., discloses a need for correcting pyrometer readings when depositing thin dielectric films in a rapid thermal processing process. Nulman et al. concludes that "the use of in situ emissivity correction systems, capable of taking into account the emissivity dependence on temperature, chamber reflectivity and pyrometer wavelength are essential for accurate temperature measurement in RTP systems". However, there is no suggestion as to how in-situ emissivity correction systems may be designed or how such in-situ emissivity correction may take place.
One proposal for compensating for changes in emissivity of the wafer as a function of thin film thickness is disclosed in a publication entitled "Broadband Pyrometry and Effects of Roughness on RTP Repeatability" published in the Proceedings of Materials Research Symposium, June, 1989, by D. Hodul. Hodul discloses the use of two pyrometers which are calibrated at different wavelengths. By using two pyrometers and a look-up table, deposition can be corrected as the film is deposited. Unfortunately, this system requires two pyrometers and two windows in the rapid thermal processing furnace, thereby increasing the cost of the rapid thermal processing system. Moreover, a complicated control algorithm is necessary for controlling power to the radiant heat sources as a function of two pyrometer readings. It is also difficult to modify an existing controller to use two pyrometers. Finally, it is known that semiconductor wafers tend to emit radiation within a narrow band of wavelengths in the infrared region. It has proven difficult to reliably control the system using two pyrometers which must measure different wavelengths within this narrow band.